System and method for modulation mapping

ABSTRACT

An apparatus for providing modulation mapping is disclosed. The apparatus includes a laser source, a motion mechanism providing relative motion between the laser beam and the DUT, signal collection mechanism, which include a photodetector and appropriate electronics for collecting modulated laser light reflected from the DUT, and a display mechanism for displaying a spatial modulation map which consists of the collected modulated laser light over a selected time period and a selected area of the IC.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 13/095,831, filed on Apr. 27, 2011, which is adivisional application of U.S. patent application Ser. No. 12/534,069,filed on Jul. 31, 2009, which is a divisional application of U.S. patentapplication Ser. No. 11/438,121, filed on May 18, 2006, which claims thebenefit of priority from, U.S. Provisional Patent Application Ser. No.60/711,822, filed on Aug. 26, 2005, the entire disclosures of which arerelied upon and incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to an apparatus and method for probingintegrated circuits using external illumination.

2. Description of the Related Art

Probing systems have been used in the art for testing and debuggingintegrated circuit (IC) designs and layouts. Various laser-based systemsfor probing IC's are known in the prior art. In these prior art systems,the DUT is driven by an electrical test signal, while a laser beam isused to illuminate the DUT. The laser beam then reflects from the DUT,and the reflection is perturbed according to the DUT's response to theelectrical test signals. The reflected beam is then converted to anelectrical signal having a waveform corresponding to the reflected beam.This waveform is displayed for the user's analysis.

While some description of the prior art is provided herein, the readeris encouraged to also review U.S. Pat. Nos. 5,208,648, 5,220,403 and5,940,545, which are incorporated herein by reference in their entirety.Additional related information can be found in Yee, W. M., et al. LaserVoltage Probe (LVP): A Novel Optical Probing Technology for Flip-ChipPackaged Microprocessors, in International Symposium for Testing andFailure Analysis (ISTFA), 2000, p 3-8; Bruce, M. et al. WaveformAcquisition from the Backside of Silicon Using Electro-Optic Probing, inInternational Symposium for Testing and Failure Analysis (ISTFA), 1999,p 19-25; Kolachina, S. et al. Optical Waveform Probing-Strategies forNon-Flipchip Devices and Other Applications, in International Symposiumfor Testing and Failure Analysis (ISTFA), 2001, p 51-57;

Soref, R. A. and B. R. Bennett, Electrooptical Effects in Silicon. IEEEJournal of Quantum Electronics, 1987. QE-23(1): p. 123-9; Kasapi, S., etal., Laser Beam Backside Probing of CMOS Integrated Circuits.Microelectronics Reliability, 1999. 39: p. 957; Wilsher, K., et al.Integrated Circuit Waveform Probing Using Optical Phase Shift Detection,in International Symposium for Testing and Failure Analysis (ISTFA),2000, p 479-85; Heinrich, H. K., Picosecond Noninvasive OpticalDetection of Internal Electrical Signals in Flip-Chip-Mounted SiliconIntegrated Circuits. IBM Journal of Research and Development, 1990.34(2/3): p. 162-72; Heinrich, H. K., D. M. Bloom, and B. R. Hemenway,Noninvasive sheet charge density probe for integrated silicon devices.Applied Physics Letters, 1986. 48(16): p. 1066-1068; Heinrich, H. K., D.M. Bloom, and B. R. Hemenway, Erratum to Noninvasive sheet chargedensity probe for integrated silicon devices. Applied Physics Letters,1986. 48(26): p. 1811; Heinrich, H. K., et al., Measurement of real-timedigital signals in a silicon bipolar junction transistor using anoninvasive optical probe. IEEE Electron Device Letters, 1986. 22(12):p. 650-652; Hemenway, B. R., et al., Optical detection of chargemodulation in silicon integrated circuits using a multimode laser-diodeprobe. IEEE Electron Device Letters, 1987. 8(8): p. 344-346; A. Black,C. Courville, G Schultheis, H. Heinrich, Optical Sampling of GHz ChargeDensity Modulation in Silicon Bipolar Junction Transistors ElectronicsLetters, 1987, Vol. 23, No. 15, p. 783-784, which are incorporatedherein by reference in their entirety.

Some of the test and debug technique used in the prior art include LIVA(Light Induced Voltage Alteration), TIVA (Thermally Induced VoltageAlteration), CIVA (Charge Induced Voltage Alteration), XIVA (ExternallyInduced Voltage Alteration), OBIC (Optical Beam Induced Current), OBHIC(Optical Beam Heat Induced Current), and OBIRCH (Optical Beam InducedResistance Change). These techniques probe the DUT (device under test)to detect a change in the characteristics of certain devices orstructures therein to thereby detect a failure or an area that is proneto fail or adversely affect the DUT's performance. According to thesetechniques, the DUT is driven by an electrical signal, while a laserbeam is used to illuminate the DUT to thereby cause either heating,carrier generation, or both. As a result, the electrical output from theDUT is perturbed, and this perturbation is detected and analyzed. Thatis, under these techniques the laser beam is used only as a perturbingagent, but the detection is done by analyzing the electrical output fromthe DUT.

FIG. 1 is a general schematic depicting major components of alaser-based system architecture, 100, according to the prior art. InFIG. 1, dashed arrows represent optical path, while solid arrowsrepresent electronic signal path. The optical paths represented bydashed lines are generally made using fiber optic cables. Probing system100 comprises a mode-locked laser source MLL 110, an optical bench 112,and data acquisition and analysis apparatus 114. The optical bench 112includes provisions for mounting the DUT 160 and includes beam optics125. The beam optics may include various elements to shape the beam,generally shown as beam manipulation optics, BMO 135, and elements forpointing and/or scanning the beam over the DUT, such as a laser scanningmicroscope, LSM 130. A computer 140 or other device may be used toprovide power and/or signals, 142, to the DUT 160, and may providestrigger and clock signals, 144, to the mode-locked laser source 110and/or the analysis apparatus 114. The analysis apparatus, 114, includesworkstation 170, which controls as well as receives, processes, anddisplays data from the signal acquisition board 150 and the opticalbench 112.

In operation, computer 140, which may be a conventional ATE (AutomatedTesting Equipment, also known as Automated Testing and Evaluation),generates test vectors that are electrically fed to the DUT. The ATEalso sends sync signal to the mode-locked laser source, which emits alaser beam. The beam optics 125 is then used to point the beam toilluminates various positions on the DUT. The beam reflects from theDUT, but the reflection is perturbed by the DUT's response to the testvectors. This perturbed reflection is detected by photodetector 136,which converts it into an analog signal. The analog signal is acquiredby the signal acquisition board 150 and is fed to computer 170, where itis displayed as a waveform corresponding to the perturbed reflectionfrom the DUT. By correlating the timeline of the waveform to that of theATE, the response of the DUT can be analyzed.

While the arrangement depicted in FIG. 1 has been used successfully inthe art, there is a constant search for new systems that can providefurther information regarding the operation and characteristics of theDUT. Accordingly, there is a need in the art for a system that willallow improved laser probing of a DUT to enable further investigation ofchip designs.

SUMMARY

Various embodiments of the present invention provide apparatus andmethod for laser probing of a DUT to provide modulation information of aselected location or of an area or the entire DUT.

According to various embodiments of the subject invention, probing of aDUT is done by applying stimulation signals to the DUT, and illuminatingthe DUT with a laser beam. The reflection of the laser beam is collectedand analyzed to provide one or more of the reflection's amplitude,intensity and phase for a selected location.

According to various embodiments of the subject invention, probing of aDUT is done by applying stimulation signals to the DUT, and illuminatingthe DUT with a laser beam. The reflection of the laser beam is collectedand analyzed to provide a spatial map of one or more of the reflection'samplitude, intensity and phase for each point in a selected area of theDUT.

Various embodiments of the present invention find faults in anintegrated circuit by analyzing signals induced on a laser beamreflected from an active region of the device. The analysis can includedetecting RF power at a certain bandwidth or bandwidths, demodulatingthe in-phase and quadrature components of an RF signal, power filteredby a matched filter, and displaying the result either at a fixedposition or by creating a spatial map of the induced effect. Thetechnique may also involve comparing signals from a known good die andfrom an inspected die.

Various embodiments of the present invention solve the problem ofisolating faults in an integrated circuit without making physicalcontact with the circuit. In particular, it can isolate resistive faultsbetween gates without requiring synchronous detection from a tester orstimulus board.

According to various embodiments of the present invention, an apparatusfor providing modulation mapping is provided. The apparatus includes alaser source, a motion mechanism providing relative motion between thelaser beam and the DUT, signal collection mechanism, which include aphotodetector and appropriate electronics for collecting modulated laserlight reflected from the DUT, and a display mechanism for displaying aspatial modulation map which consists of the collected modulated laserlight over a selected time period and a selected area of the IC.

In one embodiment of the invention, an apparatus for providingmodulation mapping is provided, wherein a spectrum analyzer is used forgenerating the modulation mapping.

In another embodiment of the invention, an apparatus for providingmodulation mapping is provided, wherein a lock-in amplifier is used forgenerating the modulation mapping.

Various embodiments of the invention also provide for a laser scanningmechanism that is operable to scan the selected area to obtain an imageof the scanned area, and is operable to scan the selected area to obtainmodulation signal from the scanned area. The generated image is thenused to normalize the modulation signal and the normalized modulationsignal is used to generate a normalized modulation mapping.

In another embodiment of the invention, a system for testing anintegrated circuit microchip using laser probing comprises a lasersource providing a laser beam; a beam optics receiving said laser beamand focusing said laser beam onto a selected spot on said microchip; aphotosensor receiving reflected laser light that is reflected from saidmicrochip and providing an electrical signal; collection electronicsreceiving the electrical signal from said photosensor and providing anoutput signal; an analysis system receiving said output signal andproviding a total power signal.

According to one aspect of the invention, a method is provided fortesting an integrated circuit (IC), the method comprising: stimulatingsaid IC with a test signal; illuminating said IC with a laser beam;collecting beam reflection from said IC; converting said beam reflectionto an electrical probing signal; selecting one or more frequencies or aband of frequencies of said probing signal; calculating at least one oftotal amplitude, total intensity, and phase of said probing signal atthe selected frequency or band of frequencies.

Other aspects and features of the invention will become apparent fromthe description of various embodiments described herein, and which comewithin the scope and spirit of the invention as claimed in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general schematic depicting major components of alaser-based IC probe system architecture according to the prior art.

FIG. 2 is a diagram illustrating a first embodiment of the presentinvention.

FIG. 3 is a diagram illustrating another embodiment of the presentinvention.

FIG. 4 is a diagram illustrating a fault between two active nodes of aDUT.

FIGS. 5 a-5 e are plots illustrating multiplication of probing andreference signals.

FIG. 6 a depicts a reflectivity map of a DUT, while FIG. 6 b depicts amodulation map generated according to an embodiment of the invention.

FIG. 7 depicts another embodiment of the present invention.

FIG. 8 depicts yet another embodiment of the present invention.

FIG. 9 is a diagram illustrating the polarization differential probing(PDP) mode for phase detection according to an embodiment of theinvention.

FIG. 10 is a diagram illustrating why the laser probing signalmodulation is intrinsically polarization sensitive for CMOS transistorsdue to the asymmetric structure of the CMOS device.

FIG. 11 is an illustration depicting a PDP optical path according to anembodiment of the invention.

FIG. 12 is a plot illustrating the benefit of detecting both returnbeams of the PDP interferometer.

FIG. 13 illustrates a further embodiment of the invention.

The invention is described herein with reference to particularembodiments thereof, which are exemplified in the drawings. It should beunderstood, however, that the various embodiments depicted in thedrawings are only exemplary and may not limit the invention as definedin the appended claims.

DETAILED DESCRIPTION

Various embodiments of the present invention provide apparatus andmethod for testing an integrated circuit. As is known, integratedcircuits include active devices, such as transistors. The subjectinvention is based on the inventor's observation that when a transistoris active, it can modulate a light beam illuminating active regions ofthe transistor. The strength of the modulation depends on thetransistor's response to a potential applied to it. Consequently, when alaser beam illuminates the active area, the reflection would bemodulated according to the active area's response to applied potential.On the other hand, when the beam illuminates an inactive area, thereflection would not be modulated. Therefore, the study of the beamreflection can provide information regarding the response of activeareas to applied potential. It should be noted, however, that while thediscussion herein refers to the use of laser beams, other light sourcesmay be used, so long as it is of sufficient energy to enable detectionwith acceptable signal to noise ratio (SNR) and is sufficientlymonochromatic to enable the interaction.

Heretofore, artisans have been studying the time-based waveform of thereflected laser light. While such studies provide useful information,the present inventor has determined that further insight can be gainedby other studies of the reflected light. More specifically, the subjectinventor has determined that information regarding the functionality ofthe DUT can be gained by studying the amplitude, intensity, polarizationrotation, and/or phase of the reflected beam at each selected locationon the DUT. The following description provides various embodiments formethods and apparatus enabling such studies.

An embodiment of the invention will now be described in details withreference to FIG. 2. In this embodiment of the invention, the DUT 260 isprovided with test signals, 242, e.g., from PC 270. As is shown bycallout 248, the signal may be a simple square wave at a selectedfrequency φ. Concurrently, laser source 202 provides a laser beam 204.In this particular example, a tunable or continuous wave (CW) lasersource may be used, although other sources may be used, such as pulsedor mode-locked laser sources. Optionally, beam splitter 206 is providedto reflect a small portion of the beam, e.g. 5%, towards optionalphotodetector 208. The output of photodetector 208 is used to monitorthe proper operation of the laser source 202. This feature isbeneficial, but is not necessary for the operation of the invention. Thelaser beam is then conditioned and directed onto the DUT 260 by the beamoptics 212. The beam optics may include beam manipulation optics, BMO235 for focusing and conditioning the beam to the desired properties.Beam optics 212 may also include elements 230 for pointing and/orscanning the beam over the DUT. Such element may be, for example, anLSM. On the other hand, rather than using pointing or scanningmechanism, the beam may be stationary and provisions made for moving orscanning the DUT. This can be achieved by, for example, structuring theDUT holder 232 to function as a controllable X-Y stage. In essence, anystructure enabling relative motion between the beam and the DUT isacceptable.

The reflected beam from the DUT is collected by the beam optics 212, andis directed towards the photodetector 236. The photodetector provides anoutput, normally an analog electrical signal, that is representative ofthe reflected light beam. The AC component (generally RF) of thephotodetector output is amplified by amplifier 264 (e.g., an RFamplifier), and the amplified signal is provided to a spectrum analyzer246. The user may select a specific frequency or a specific band offrequencies, and the spectrum analyzer output the total power, TP,received in the specified frequency or band of frequencies. Morespecifically, as is shown in callout 244, the amplified signal has aspread of power P over its frequencies f. The user may select a singlefrequency f₀, and the spectrum analyzer would provide the power P₀ ofthe signal in the selected frequency. Alternatively, the user may selecta band of frequencies, Δf, in which case the spectrum analyzer wouldoutput a value for the total energy for the selected band. It should beappreciated that the frequency f₀ at which peak power is observedrelates to the frequency φ of the input signal to the DUT. When thereflected power is given by P_(R), the power spectral density S=S(w),the frequency is w₀, and the width of the frequency band is Δw, thetotal integrated power is given by:

∫_(W) ₀ ^(w) ⁰ ^(+Δw/2) P _(R)(w)dw  Eqn. 1.

The total power output, TP, from the spectrum analyzer 246 is collectedover a specified period of time, e.g., by frame grabber 276, and theresulting value is provided to the user, e.g., on display 274 of PC 270.The collection over a period of time can be beneficially obtained byintegrating the output of the spectrum analyzer over a specified periodof time. Notably, as can be understood, rather than presenting thewaveform as is done in the prior art, in this embodiment a spectrumanalyzer is used to determine the amount of power in a selectedfrequency or a band of frequencies of the reflected laser beam. Byproviding this value for each selected location on the DUT, thefunctionality of various devices of the DUT can be analyzed. As can beseen, to perform this analysis a simple test signal, such as a simplesquare wave is sufficient. Consequently, for this testing there is noneed to employ an expensive ATE. Rather, one can use the PC 270 togenerate the test signal.

As also shown in FIG. 2, the DC part of the photodetector signal can beinput to a video amplifier. The signal of the video amplifier can beused to generate a video image to enable navigation over the DUT.Additionally, according to a feature of this embodiment, the signal fromthe video amplifier can be used to normalize the signal from thespectrum analyzer. This feature is particularly beneficial when ascanning of a selected area of the DUT is performed, so as to generate amodulation map. The modulation map is a spatial map upon which the totalpower at a selected frequency or band of frequencies is indicated foreach location of the scanned area. The signal from the video amplifiercan be used to generate a normalized modulation map, i.e., a spatial mapupon which the total power at a selected frequency or band offrequencies is indicated for each location on the scanned area, asnormalized by the video image signal.

FIG. 3 depicts another embodiment of the present invention. Notably, theembodiment of FIG. 3 is similar to that of FIG. 2, except that thespectrum analyzer has been replaced with a lock-in amplifier. Therefore,the description of the embodiment of FIG. 3 is limited to the feature ofthe lock-in amplifier, and all elements similar to that of FIG. 2 havethe same reference characters as in FIG. 2. As shown, the signal fromthe AC amplifier is fed to the lock-in amplifier. Additionally, areference signal is also fed to the lock-in amplifier. Generally, if thereflected detection signal is given by I_(R)(t), and the referencesignal is given by L(t), both signals are multiplied by each other andthen integrated, so that the output of the lock-in amplifier is givenby:

$\begin{matrix}{\frac{1}{T}{\int_{0}^{T}{{L(t)}{I_{R}(t)}{{t}.}}}} & {{Eqn}.\mspace{14mu} 2}\end{matrix}$

The reference signal is of frequency θ, which would typically be set tobe the same as that of the AC signal output by the AC amplifier, butphase shifted by a selected amount Δφ. As can be understood, thefrequency θ relates to the frequency φ of the test signal. As shown inEqn. 2 and in FIG. 3, in the lock-in amplifier the signals are firstmultiplied by each other, 241, and the resulting product is integrated,243. The output is then collected by the frame grabber 276, so that atotal power value is resulted for each defined period of time. Usingthis value, the activity of the DUT can be analyzed. As in theembodiment of FIG. 2, total power values for multiple locations of aselected area of the DUT can be obtained and then spatially plotted toprovide a modulation map, i.e., a spatial map of the selected area withtotal power value indicated for each location. Also, as in theembodiment of FIG. 2, an image of the DUT can be obtained and used tonormalize the power values, so that a normalized power map can beconstructed.

The embodiment of FIG. 3 can also be used to find faults in the DUT bylooking at phase shift in the reflected light. More specifically, when aconnection from a particular node to an active element is faulty, it cancause delay in the test signal reaching that active element. This isillustrated in FIG. 4, wherein a fault 405 is depicted as a resistiveelement between active elements 400 and 410. Under such circumstances,the active device 410 would respond once the signal is received, but theresponse would be delayed in time. Such faults cannot be easily detectedusing conventional techniques, such as OBIRCH or static emission, andwould require exact synchronization with a tester, such as an ATE, to bedetected using dynamic emission testers. However, using the embodimentof FIG. 3, no synchronization is needed and the test signal may be asimple sine or square wave signal.

When the test signal has a periodic format, such as a sine or squarewave signal, the faulty connection would cause a delay in the signalreaching the device, and the device would respond in a delayed manner.Consequently, if a laser beam illuminates the device, the reflected beamwould be modulated periodically at the same frequency as the testsignal, but it would be phase shifted. This phase shift can be detectedusing the embodiment of FIG. 3, since a phase shift in the input signalto the lock-in amplifier would cause a change in the product signalresulting from the multiplication of the input and reference signal.Lock-in and frequency demodulation techniques are well known in the art.

The operation of the embodiment of FIG. 3 can be understood from thefollowing description with reference to the plots of FIGS. 5 a-5 e.Assuming that the test signal is a sine wave of frequency θ (ωt) asshown in FIG. 5 a, then the reflected laser beam will be modulated atthe same frequency so that the signal input to the lock-in amplifieralso has frequency θ. If the reference signal is also a sine wave offrequency θ and same phase as the input signal, then the multiplicationof the input and reference signals results in the signal shown on plot 5b. If, however, the reference signal is shifted by π/2, then theresulting product is as shown in FIG. 5 c, which will integrate to zero.Therefore, it is beneficial to set the reference signal at the samefrequency, but shifted by π/4, so that the resulting product is as shownby FIG. 5 d. Then, whenever the laser beam is placed over an activeregion, the resulting product will be as shown in FIG. 5 d and theintegration of that product over a specified period will result in agiven value. This value can be displayed, compared to a known valuecalculated or experimentally obtained from a known properly functionalregion, or compared to values obtain from other active regions on thesame or different DUT. If the value deviates by a more than an allowableamount, then the active region can be flagged as faulty.

As explained previously, when a connection to an active region isfaulty, it may cause delay in the reception of the test signal by theactive region. Under such circumstances, the RF signal on the reflectedlaser beam would be phase shifted. As shown in the plot of FIG. 5 e, thereference signal is kept at π/4 shift from the proper, un-shifteddetector signal. However, if the signal from the reflected beam is alsoshifted, in this example by π/1.2, then the resulting plot shiftsvertically, which would cause a change in the integrated value. Thischange can be used to detect faulty active regions on the DUT.

FIG. 6 a depicts an image of transistors in an integrated circuitobtained using the inventive system to map reflected light intensity,i.e., the image is constructed using the output from the videoamplifier. In this image, which is generally a reflectivity map, highergrayscale values indicate higher reflectivity regions. FIG. 6 b, on theother hand, depicts modulation map, showing activity of various deviceswithin the DUT at a particular frequency of interest. In this particularexample, the modulation data was acquired simultaneously with the imageof FIG. 6 a and a spectrum analyzer was employed. In this modulationimage higher grayscale values indicate larger modulation signal, i.e.,active regions. In this example the DUT was driven with 100 MHz clock,and the spectrum analyzer was tuned to 100 MHz frequency.

FIG. 7 depicts another embodiment of the subject invention. As can beunderstood from the above description, the embodiments of FIGS. 2 and 3rely mainly on amplitude modulation of the laser beam. The embodiment ofFIG. 7 is designed to detect phase modulation of the laser beam.Therefore, while the system of FIG. 7 is similar to that of FIGS. 2 and3, the embodiment of FIG. 7 also employ a phase to amplitude converter292. The output of the phase to amplitude converter 292 is applied tothe photodetector 236 and is amplified by RF amplifier 264. Theamplified signal is then provided to the analyzer 248, which may be aspectrum analyzer, a lock-in amplifier, etc., as explained with respectto FIGS. 2 and 3. The phase to amplitude converter 292 may be anyconventional converter, such as a Michelson interferometer, spatialdifferential interferometer, time differential prober, etc. Such systemsare described in U.S. patent application Ser. No. 11/169,423, commonlyassigned to the present assignee, which is incorporated herein byreference in its entirety.

FIG. 8 illustrates another embodiment of the subject invention. Theembodiment of FIG. 8 is similar to that of FIG. 7, except that apolarization differential probing (PDP) optics, 800, is used as a phaseto amplitude converter. The PDP optics is described in more detailsbelow. The system operates as follows. The DUT 860 is stimulated withtest signals from stimulus 862, which may be a stand alone equipment orpart of computer 870. If the stimulus 862 is a stand alone device, it isbeneficial to provide a communication link between the computer 870 andthe stimulus 862, as shown. While the DUT is stimulated, laser source802 provides laser beam 804. Optionally, a small portion of the laserbeam is deflected by polarizing beam splitter 806 and is detected byphotodetector 808 so as to monitor the operation of the laser source802.

The remainder of the laser beam passes through polarizing beam splitter803 and enters a Faraday rotator 805. The beam then passes throughanother polarizer beam splitter 806. Upon exiting beam splitter 806, thebeam is actually composed of two orthogonal polarization states, or twoco-located beams polarized at 90° to each other. Both beams then enter ascanning mechanism 830, which may be an LSM, AOD (acoustic-opticaldeflector), scanning lens, tube lens, etc. In this embodiment, an LSM830 is used, which is controlled by computer 870. The beams then pass avariable retarder 807, which retards only one of the beams. This beamretardation is shown schematically in FIG. 8 by the double-headed arrow.Both beams then pass through the objective lens 809 and enter the DUT860. The objective lens 809 may be a simple single lens, or acombination of lenses optionally including a solid immersion lens (SIL)811.

One or both beams may be modulated by the DUT upon reflection; however,as will be explained further below, the inventors have observed that thebeams are modulated by a different amount. The reflected beams passthrough the objective lens 809 and enter the variable retarder 807,wherein an additional retardation is introduced. After passing thescanner 830, half of each beam is deflected by the polarizer beamsplitter 806 towards photodetector 836, and the remaining half is passedthrough the Faraday rotator 805 and is deflected by the polarizer beamsplitter 803 towards photodetector 837. In this embodiment, eachphotodetector is an avalanche photodiode (APD). The output fromphotodetector 836 is applied to bias T1, while the output ofphotodetector 837 is applied to bias T2. Optionally, the DC component ofthe signal is routed to video amplifier 866 and to frame grabber 876 togenerate an image of the DUT. Another option is to have the DC componentof the signal of bias T2 also routed to video amplifier 867 and to framegrabber 876 to generate another image of the DUT. The AC part of eachsignal is amplified by RF amplifiers 864, 868, and the signal is appliedto selector 877. Selector 877 provides an output that may be asummation/difference of both signals, the signal of T1 only, or thesignal of T2 only. The output of the selector 877 is then applied to theanalyzer 848, which may be a spectrum analyzer, lock-in amplifier, etc.,as explained with respect to FIGS. 2 and 3.

FIG. 9 is a diagram illustrating the polarization differential probing(PDP) mode for phase detection, as can be employed in the embodiments ofFIGS. 7 and 8. A laser beam 920 from a single modulated laser source issplit into two orthogonally polarizes laser beams 922, 924, by beamoptics 925. Both beams 922, 924 traverse the same optical path, but haveorthogonal linear polarization states as depicted by the dots andarrows. Unlike conventional polarization schemes where two differentlocations on the DUT are illuminated by the reference and the probingbeams, in this embodiment both beams are made to incident on the samepoint 932 on the DUT 910. Also, unlike the polarization scheme of theprior art where the two beams are split in time, in this embodiment bothbeams are made to be incident on the DUT 910 at nominally the same time.For optimal effect, the polarization directions of the beams 922, 924are aligned with the transistor gate width and length directions in theDUT 910. This polarization difference results in phase modulationdifferences between the two beams after DUT interaction, as will bedemonstrated below with reference to FIG. 10. The two beams 922, 924 aremade to interfere 926 after their DUT interactions in order to converttheir phase variations into amplitude variations that can be detectedusing photosensors. A differential detection scheme can be optionallyemployed to increase signal modulation.

FIG. 10 is a diagram illustrating why the laser probing signalmodulation is intrinsically polarization sensitive for MOS transistorsdue to the intrinsic asymmetry of the MOS device. A laser beam 1020 isincident from the bottom (through the silicon substrate). For maximumFranz-Keldysh effect (electro-absorption/refraction), the laser beam'spolarization state should be aligned parallel to the direction of thestrong modulating fields in the gate/drain regions of the transistor,i.e., along the gate length direction (along E_(x) in FIG. 10). On theother hand, the Plasma-Optical effect requires the laser beam to drivethe charge carriers induced under the gate. Since the charge carriersare less constrained in the gate width direction, a laser beam polarizedalong E_(y) should be most sensitive to this effect. In practice, thelaser beam is found to be most strongly modulated when its polarizationvector is aligned along E_(y). Using these effects, the presentinventors realized that, unlike the prior art interferometricarrangements, the reference beam need not traverse a reference pathseparate from the probing path. Rather, it is possible to have bothreference and probing beams traverse the same path and be pointed ontothe same point on the DUT at nominally the same time, while stillobtaining an interferometric effect that correlates to the DUT'sresponse to the stimulating signal. Of course, it should be appreciatedby artisans that the use here of “reference beam” and “probing beam” isfor convenience purposes only, since it should be apparent that unlikethe prior art, here both beams illuminate the exact same area to beinvestigated at nominally the same time and, therefore, in this sense,each beam can be both a reference and a probing beam.

FIG. 11 is an illustration depicting a common-path PDP opticalarrangement according to an embodiment of the invention. While inactuality the incident beam and the reflected beam traverse the sameelements, for better understanding the illustration is divided into twooptical paths, showing each optical element twice. The top path is theincident beam path originating from the laser source, while the bottompath is the reflected beam path. The incident beam travels from left toright in the illustration. Along the beam path, vertically orientedarrows indicate a vertically polarized beam while dots indicate ahorizontally polarized beam. A tilted arrow indicates a beam that islinearly polarized at some angle off the vertical. Spatial separationbetween a dot and an arrow depicts a phase difference between the twobeams.

The beam from the modulated laser source enters the first polarizingbeam splitter PBS1 so that part of the beam is deflected towards lightsensor 1110. This deflection may be set at 5% or so. The output of thesensor 1110 is used to monitor the beam's intensity and is not part ofthe PDP optics, but is rather an optional intensity monitor. Theremaining part of the beam that passes through the first PBS cube (PBS1)enters the second polarizing beam splitters PBS2, which is oriented topass only a vertically oriented beam. The beam's polarization state isrotated a predetermined amount so as to generate a rotated polarizedbeam that is an equivalent of a superposition of a vertically polarizedbeam and a horizontally polarized beam. In this example, the beam isrotated 45 degrees from the vertical by the action of the Faradayrotator (FR) and the third PBS cube (PBS3) is oriented to transmit therotated beam. Consequently, at this stage the beam is the equivalent ofa superposition of a vertically polarized beam and a horizontallypolarized beam, both beams equal in amplitude and in phase with eachother. If the amplitude of the beam should not be set to equal, therotation should be to a different angle. The dashed callout circles inthe incident beam path, between PBS3 and VR, indicate the equivalencebetween a 45-degree polarized beam and two in-phase, equal amplitudebeams, one polarized vertically and the other horizontally. As can beunderstood, for certain application the rotation may be to other than 45degrees, in which case the equivalence would be of two, in-phase beams,one polarized vertically and the other horizontally, but havingdifferent amplitude.

The two beams then enter the variable retarder VR. The fast and slowaxes of the variable-retarder (VR) are aligned along these vertical andhorizontal polarization directions. Thus, after passage through the VR,the beam consists of two spatially coincident, equal-amplitude,orthogonally polarized beams that are phase-shifted (retarded) withrespect to each other by a small amount (nominally, π/4). This isindicated in the illustration by the dot being slightly behind thevertical arrow, representing a retardation of the horizontally polarizedbeam relative to the vertically polarized beam. The two beams are thenfocused onto the same point on the DUT by the objective lens OL. The DUTis oriented such that the polarization directions of these two beams arealigned with the length and width directions of the transistor gates.Interaction with the DUT phase modulates one of the beams relative tothe other by a small amount. In this manner, the beam being modulated bythe DUT may be thought of as the probing beam, while the other beam maybe thought of as the reference beam. Of course, unlike prior artinterferometers, here none of the beams traverses a reference opticalpath, but rather both beams traverse the identical path to the probinglocation. Therefore, as noted before, in this sense there is noreference beam and probing beam, but for convenience one may refer toone beam as the reference beam and the other as the probing beam.

After the beams are reflected by the DUT (FIG. 11, bottom) the twolinearly polarized beams retrace their path to the VR. The phasemodulation induced on the beams by the DUT is typically much smallerthan the phase shift induced by passage through VR and so is notexplicitly shown here. Passing through the VR introduces an additionphase-shift between the two returned reference and probing beams (nownominally phase-shifted by π/2). At PBS3, a portion of each beam isreflected and sent to photosensor 1120, and the other portion istransmitted. The reflected portions interfere, since they are now in thesame polarization state, and generate the reflected-A signal. In oneembodiment, only one photosensor 1120 is used and its output iscollected by the receiver electronics and analyzed, as described in moredetail below. According to another embodiment, the transmitted portionsare deflected out of the beam path via the action of FR and PBS2 so asto be detected by a second photosensor 1130. The transmitted halves alsointerfere since they are also in the same polarization state, generatingthe reflected-B signal. The reflected-B signal is collected by thereceiver electronics and is analyzed, as described in more detailsbelow.

An analysis of the interference condition shows that the intensity ofthe reflected-A signal is given by:

R(A)∝(E _(x)/√2)²+(E _(y)/√2)²−2[(E _(x)/√2)(E _(y)/√2)] Cos(θ+δ)  Eqn3.

Here the θ term accounts for the static phase shift introduced by thedouble-pass through the variable rotator, VR, while the δ term is thesmall varying relative phase shift of the two beams resulting from theinteraction with the DUT as it undergoes testing. Similar analysis forthe reflected-B signal results in:

R(B)∝(E _(x)/√2)²+(E _(y)/√2)²+2[(E _(x)/√2)(E _(y)/√2)] Cos(θ+δ)  Eqn.4.

Eqn. 3 and Eqn. 4 are plotted in FIG. 12. From this plot, it is clearthat setting the static phase shift, θ, to ±π/2 (quarter waver conditionfor the round trip beam) gives maximum sensitivity to changes in 6 andmakes the two signals nominally equal in intensity. Under thiscondition, Eqn. 3 and Eqn. 4 simplify, respectively, to:

R(A)∝(½)E _(x) ²+(½)E _(y) ² +E _(x)E_(y) Sin(δ)  Eqn. 5.

and

R(B)∝(½)E _(x) ²+(½)E _(y) ² −E _(x)E_(y) Sin(δ)  Eqn. 6.

Then,

R(A)−R(B)∝2E _(x) E _(y) Sin(δ)  Eqn. 7.

Thus, in principle, subtracting the two reflected signals eliminatestheir large DC component along with any noise it carries, such as fromlaser power variations, while doubling the signal modulation.Consequently an improved signal to noise ratio (SNR) is provided whenusing this differential signal detection mode. In practice, digitallydividing one signal by the other instead of subtracting them isperformed because it is more tolerant of unbalanced reflected signals.

As can be understood, since both reference and probing beams traversethe identical optical path and are pointed to the same location on theDUT at nominally the same time, it means that this scheme has betterphase noise immunity then prior art interferometric systems. Notably,both beams are subject to the same vibrations and optical losses. Thiseliminates or reduces the need for active vibration compensation, pathlength matching, and power matching of the two arms of theinterferometer. Additionally, there is no need to find a second locationfor the reference beam for each location tested. Rather, both beams arealways pointed at the location to be tested. Accordingly, there is alsono need to introduce separate spatial control of the reference andprobing beams. There is also no need for complicated beam time-shiftingand unshifting optics and no coupling of signal strength with timeresolution. Consequently, the inventive common-path PDP arrangement canbe used in multiple applications where phase detection is needed andprovides drastic reductions in vibration noise, reduces the systemcomplexity, and simplifies the setup of the system.

Referring back to FIG. 8, in the embodiment depicted two photosensors,APD1 and APD2 are used. For an increase in light intensity of theprobing signal, the two photodiodes may be used in a differentialdetection mode. According to the embodiment of a differential detectionmode, one diode produces a positive going signal at its amplifier input,while the other one produces a negative signal. This can be done bynegatively biasing one photodetector, say APD1, while positively biasingthe other photodetector, as is shown schematically in FIG. 8. In thisembodiment, the bias is to about −60V and +60V, respectively. Summingthe two output signals, e.g., by selector 877, produces an enhanceddifferential signal. Monitoring the current of both APD's assists inchecking the balance of the PDP optics, as the variable phase plate ofthe PDP can be adjusted until the same current is observed by the twocurrent monitors 898 and 894.

During navigation or for generating a reflectivity map, the signal fromonly one APD is required. As is shown, the signal from each photosensoris also sent to a video amplifier, which provides a video out signal forprocessing and display. However, it is possible to use a signal fromonly one photodetector and one video amplifier to generate areflectivity image. Advantageously, in order to provide contrastcontrol, the variable retarder may be varied to tune the retardation sothat the image contrast is varied to the desired result. Additionally,imaging may be performed using both APD's and the resulting imagessubtracted from each other so as to obtain a difference image.

In the various embodiments disclosed where two APD's are used, the APD'sgain may be advantageously controlled to, first, balance the APD'sresponse and, second, to improve the imaging. Using a controllablevariable power supply that is manually or automatically controlled, thevoltage/gain response of each APD can be determined. Then, using thelearned voltage/gain response, the gain of each APD can be controlled toa desired value by selecting the appropriate voltage on thecorresponding power supply. For balancing the system, the followingprocedure may be used. First, the voltage of the power supplies of eachAPD is set to result in the same gain provided by both APD's, therebybalancing the output of the APD's. Alternatively, if a different gaincircuitry is used, it should be balanced to obtained balanced output.Then, the variable wave plate is adjusted until the current output ofboth ADP's is the same, thereby balancing the optical path. The variablegain can also be used for improved imaging. For example, when the imagescan goes from a relatively dark area to a relatively bright area, thegain of the APD's may be reduced so as not to saturate the image.Conversely, when moving from a bright area to a dark area, the gain maybe increased to provide improved contrast and detail.

FIG. 13 depicts a block diagram of another embodiment of the invention,wherein the signals are digitized individually. This allows the signalsto be summed, subtracted, divided, etc. It also gives greaterflexibility. For example, the ratio of reflected A/reflected B givespure PDP signal, but this arrangement also enables other ratios, such asreflected A/Incident pick-off and/or reflected B/Incident pick-offsimultaneously. The user may try various signals and then chose whichprovides the best result for a particular investigation.

In the embodiment of FIG. 13, a computer, such as a programmed personalcomputer PC 1300 is equipped with a digital signal processing card DSP1310 and a frame grabber 1320. In this embodiment, three channels, CH1,CH2, and CH3, provide signals to the DSP 1310. Channel CH1 is optionaland is used to monitor the operation of the laser source. Channel 1comprises photosensor 1331 which receives a light signal from a fiberoptics and outputs an electrical signal. The electrical output of thephotosensor 1331 is applied to current to voltage converter 1341 whichhas variable transimpedance gain control. The output of the current tovoltage converter is then applied to a gated integrator 1351, which isresponsive to a gate-on signal. The signal from the gated integrator1351 is applied to a variable gain/offset circuit 1361 to enablegain/offset control. The signal from the gain/offset circuitry 1361 isthen applied to a digital/analog converter DAC 1371 to convert theanalog signal into a digital signal. Variable gain/offset circuit 1361allows the signal applied to DAC 1371 to be within the input range ofthe DAC. In this embodiment, a 14 bit DAC converter with internalsample/hold feature is used. The digital output is then provided to theDSP card for processing.

As is shown in FIG. 13, channel CH1 is used to monitor the output of thelaser source and it receives the signal from the pick-off fiber asexplained above. Channels CH2 and CH3, on the other hand, are used forthe probing and receive the reflected signals Reflected-A andReflected-B, respectively. Additionally, one of the channels, in thisillustration channel CH3, is also used for imaging by applying theoutput of its current to voltage converter to a video amplifier 1380,the output of which is applied to the frame grabber 1320. The quality ofthe image may be adjusted by a brightness/contrast (or black level/gain)signal applied to the video amplifier.

The construction of channels CH2 and CH3 can be identical. Each channelreceives signal from a fiber optics and a photodetector, 1332, 1333,converts the optical signal to an analog signal. The analog signal isamplified by the RF amplifier, 1352, 1353, and is input to an analyzer,1362, 1363. The analyzer may be a spectrum analyzer as explained withrespect to FIG. 2 or a lock-in amplifier, as explained with respect toFIG. 3. The output from the analyzer is digitized and supplied to theDSP card 1310. Notably, in this embodiment the signal from the analyzeris input to the DSP card, as it may provide a higher resolution than theframe grabber. It also allows for the beam to be set in a fixed x-yposition while data is being acquired for a single spot on the DUT asthe DUT is being stimulated with variable stimulus signal. This can bedone repeatedly for various locations on the DUT. Also, when modulationdata is acquired for various different stimulus signals, a series ofmodulation maps can be generated, one for each modulation signal. Theseries of maps can also be plotted into a 3-D plot with the DUT stimulusparameter plotted in, say, the Z-direction. Further investigations canbe performed by generating cross sections of the 3-D plots.Additionally, the output of each analyzer can be input to more than onechannel. This enables, for example, to provide simultaneous maps ofamplitude and phase over the selected area of the DUT. Alternatively,one channel, say CH2 can be used to generate a power map, while theother to generate a phase map.

While the invention has been described with reference to particularembodiments thereof, it is not limited to those embodiments.Specifically, various variations and modifications may be implemented bythose of ordinary skill in the art without departing from theinvention's spirit and scope, as defined by the appended claims. Forexample, it should be understood that all of the embodiment describedherein can be used for testing by comparing the resulting probing signalto a signal of a known device, or to a signal obtained from otherdevices on the same or different DUT. Additionally, all of theabove-cited prior art references are incorporated herein by reference.

What is claimed is:
 1. A laser probing system for testing an integratedcircuit microchip having silicon substrate and active devices therein,comprising: a laser source providing a laser beam; beam opticsconfigured for receiving said laser beam and focusing said laser beamthrough the silicon substrate and onto at least one active device onsaid microchip; a photosensor receiving modulated laser light that isreflected from said microchip after being modulated by the activedevice, and providing a corresponding electrical signal; collectionelectronics receiving the electrical signal from said photosensor andproviding an output signal; an analysis system receiving said outputsignal over a specified period of time and providing a total powersignal corresponding to total radiation power of the modulated laserlight received by the photosensor over the specified period of time;and, a display presenting the total power signal.
 2. The system of claim1, further comprising a scanner for scanning the laser beam over aselected area of the microchip and wherein the display presents aspatial modulation map which corresponds to total power signal ofcollected modulated laser light over a selected time period and aselected area of the microchip.
 3. The system of claim 1, wherein thedisplay presents the total power corresponding to modulated laser lightreflected from a fixed position of the microchip.
 4. The system of claim1, wherein said active device comprises a transistor having a gate, andwherein said beam optics further comprises a polarizer configured topolarize said laser beam in alignment with the gate of said transistor.5. The system of claim 4, wherein the polarizer is configured topolarize the laser beam such that the polarization is aligned parallelto length direction of the gate.
 6. The system of claim 1, furthercomprising a video amplifier configured for receiving a DC part of theelectrical signal output by the photosensor, and generating signal for avideo image.
 7. The system of claim 1, wherein said analysis system isconfigured to normalize the output signal and use the output signal togenerate a normalized modulation mapping.
 8. The system of claim 1,wherein said analysis system comprises a selector, enabling selection ofa frequency or a band of frequencies, and wherein said total powersignal corresponds to total power at a selected frequency or a selectedband of frequencies.
 9. The system of claim 8, wherein said analysissystem comprises a spectrum analyzer.
 10. The system of claim 1, furthercomprising: a scanner for scanning the laser beam; a frame grabberreceiving said total power signal; and, a computer, said computergenerating a spatial modulation map indicating total power value forvarious locations on said integrated circuit.
 11. A laser probing systemfor testing an integrated circuit microchip having silicon substrate andplurality of semiconductor devices formed therein, comprising: amicrochip holder configured for supporting the microchip and enablingapplication of test signals to the microchip; a laser source providing alaser beam; beam optics configured for receiving said laser beam and,while the test signals are applied to the microchip, focusing said laserbeam onto a selected area of the microchip that includes at least onesemiconductor device; a photosensor receiving modulated beam reflectedfrom said selected area after being modulated by the semiconductordevice in response to the test signals, and providing a correspondingelectrical signal indicative of the semiconductor device's response tothe test signals; collection electronics receiving the electrical signalfrom said photosensor and providing an output signal; an analysis systemreceiving said output signal over a specified period of time andgenerating modulation mapping corresponding to the selected area of themicrochip.
 12. The system of claim 11, wherein the beam optics isconfigured to focus said laser beam through the silicon substrate ofsaid microchip.
 13. The system of claim 11, wherein said analysis systemcomprises a spectrum analyzer.
 14. The system of claim 13, wherein saidanalysis system further comprises an integrator integrating the outputof the spectrum analyzer over a specified period of time.
 15. The systemof claim 11, wherein said analysis system comprises a lock-in amplifier.16. The system of claim 15, further comprising a reference signalgenerator providing a reference signal, said reference signal having afrequency similar to that of said output signal, and having a differentphase from that of said output signal.
 17. The system of claim 16,further comprising a frame grabber.
 18. The system of claim 11, furthercomprising a digital signal processor.
 19. The system of claim 11,wherein said beam optics further comprises phase to amplitude converter.20. The system of claim 11, wherein said semiconductor device comprisesa transistor having a gate, and wherein said beam optics furthercomprises a polarizer configured to polarize said laser beam inalignment with the gate of said transistor.
 21. The system of claim 11,wherein said semiconductor device comprises MOS(metal-oxide-semiconductor) device.
 22. A method for probing anintegrated circuit (IC) formed in a silicon substrate, comprising:stimulating said IC with a test signal; generating a laser beam andseparating the beam into a vertically polarized component andhorizontally polarized component; illuminating a selected area of saidIC with the laser beam, through the silicon substrate, such that one ofthe vertically polarized component and horizontally polarized componentis aligned to be modulated primarily by Franz-Keldysh effect within saidIC, while the other of the vertically polarized component andhorizontally polarized component is aligned to be modulated primarily byplasma-optical effect within said IC; collecting beam reflection fromthe selected area of said IC; converting said beam reflection to anelectrical probing signal; analyzing said electrical probing signal todetermine modulation of the laser beam by said IC; and, displaying amodulation map on a monitor.
 23. The method of claim 22, furthercomprising separating the electrical probing signal into a DC componentand an AC component and using the DC component to perform at least oneof: generating an optical image of said selected area of the IC from theDC component; and, normalizing the modulation map using the DCcomponent.
 24. The method of claim 22, wherein analyzing said electricalprobing signal comprises calculating at least one of a total amplitude,a total intensity, polarization rotation, and a phase from saidelectrical probing signal at a selected frequency or band offrequencies.